Angular Inflow Research Articles

Computational study of flow incidence effects on the aeroacoustics of low blade-tip Mach number propellers

This paper presents a computational study of flow incidence effects on the aeroacoustics of a propeller operating at low blade-tip Mach numbers. The numerical flow solution is obtained by using the Lattice-Boltzmann/Very Large Eddy Simulation method, while far-field noise is computed through the Ffowcs-Williams & Hawkings' acoustic analogy applied on the propeller surface. The presence of an angular inflow leads to: (i) the radiation of tonal loading noise along the propeller axis; (ii) the increment/reduction of the sound pressure level in the region from/to which the propeller is tilted away/towards. However, contrarily to propellers operating at high blade-tip Mach numbers, the noise directivity change is found to be governed only by the rise of periodic unsteady loadings, with the modulation of the strength of the noise sources on the blade, associated to the periodic variation of the observer-source relative Mach number (in the blade reference frame), being negligible. Finally, thickness noise and turbulent boundary-layer trailing-edge noise did not show a significant directivity variation due to the propeller yaw angle change.

On a Generalized Multipole Expansion with Application to Propeller Design Synthesis

The radiation of sound by surfaces in motion in a non-uniform flow, including the effects of reflections from obstacles on noise, is specified by an extension of the Kirchhoff integral that leads to a generalized multipole expansion that extends the classical series of spherical harmonics to account for the effects of ( i) the moving medium and ( ii) the rotating sound sources. The corresponding radiation integrals are evaluated analytically for an arbitrary source distribution along the blades of a propeller at an angular inflow. The effects of the incident flow and propeller rotation on the amplitude and phase of sound, e.g. through the retarded time, lead to an extension of the generating function for Legendre polynomials; this provides the representation of the acoustic field as the generalized multipolar series. Each generalized multipole sound field is shown to consist of a fundamental blade passng frequency (BPF) plus all harmonics, with directivities specified by integrals of Bessel functions, that are evaluated analytically. The theory is validated by comparison with noise measurements using two different model propellers in two distinct aeroacoustic wind tunnels. The experimental results are used to illustrate the principle of propeller noise synthesis, relating the radiated sound field to the sound source distribution.

Sound from a propeller at angle of attack: a new theoretical viewpoint

Accurate prediction of propeller noise is required to satisfy airplane noise certification regulations and environmental concerns. Propellers typically operate with their shafts at some angle to the flight direction but this factor has been ignored in noise calculations until recently. In this paper, far field noise formulas are devel­oped by applying the free-space Green’s function for the convected wave equation to loading sources on a propeller whose shaft may be at an angle to the flight direction. The resulting inflow angularity influences noise in two distinct ways. First, loading is modulated causing generation of more efficient radiation modes. Second, the radiation modes themselves are modified, causing a further efficiency increase. The first effect is well known; the second effect has only recently been recognized and is the focus of this paper. The noise formulas exhibit the same spinning mode behaviour seen in previous analyses but with higher levels radiated into the crossflow. Since the modes are no longer purely spinning, the term ‘wobbling mode’ has been coined to describe their behaviour. This paper summarizes the earlier literature on this subject, explores the radiation formulas in detail, and presents some calculations that illustrate the magnitude of the angular inflow effect. It is shown that the inflow angle influences modal efficiency only through changing the Mach number of the source relative to the observer.

Unsteady airloading panel method for propfans

An unsteady, linear, compressible, three dimensional lifting surface panel method is presented for use with propellers and prop-fans. Both gust and blade vibration problems can be treated. Derivation of the theory is based on the acceleration potential method and can be considered a generalization of the analogous wing theory to include rotation and multiple blades. As with the wing methods, the integral equation is discretized and solved by matrix inversion. Sample calculations are use to explore the difference between 2D and 3D results and the significance of unsteadiness in angular inflow calculations for propellers. It is shown at the once-per-revolution frequency of propellers, that quasi-steady methods should be adequate. At the high frequencies of interest in noise analysis, 2D strip methods give adequate results away from blade tips. For flutter, frequencies lie in an intermediate range where a full 3D unsteady technique seems to be required.

Noise field of a propeller with angular inflow

The propeller blade forces are replaced by doublet and quadrupole distributions for the acceleration potential. An essential step in the solution is the derivation of a near-field and a far-field formula for the pressure due to a point force in arbitrary motion within a uniformly moving acoustic medium. The discrete frequency spectrum and the three-dimensional directionality of the noise radiated from a propeller with angular inflow are obtained by a conventional Fourier series expansion of the signature. The final analytical formulas can be evaluated with simple calculation.